Coincidence Doppler Broadening Spectroscopy Research Articles

Coincidence broadening doppler spectroscopy applied to elemental oxides

Abstract Coincidence Doppler Broadening Spectroscopy (CDBS) was employed to analyze ten simple oxides and compare them to their corresponding non-oxidized pure elements to identify the spectral signature attributable to oxygen. Despite variations among the oxidized elements, the resulting spectra exhibited consistent features—when referenced against the oxidized elements—that can be attributed to oxygen. Furthermore, CDBS spectra referenced against pure aluminum, a widely used independent standard, also revealed a prominent characteristic signal indicative of oxygen that can serve as a fingerprint for its presence. Comparisons with CDBS spectra of Group XVIa elements (Te, Se, and S) were also conducted to further validate the oxygen-related features. This methodology offers a valuable approach for discerning oxygen contributions in complex materials, even when the CDBS spectra are intricate and involve diverse compositions.

Surface and near-surface positron annihilation spectroscopy at very low positron energy

We present a monoenergetic positron beam specifically tailored to the needs of (near-) surface positron annihilation spectroscopy. The Setup for LOw-energy Positron Experiments (SLOPE) comprises a high-activity 22Na source, a tungsten moderator, electrostatic extraction and acceleration, magnetic beam guidance, as well as an analysis chamber with a movable sample holder and a γ-ray detection system. The tungsten moderator foil, biased between 0 and 30 V, in combination with the HV-biasable sample holder, enables positron implantation energies between 3 eV and 40 keV. At low energies (< 20 eV), the count rate typically amounts to 4400 counts per second, and the beam diameter is smaller than (12±3) mm. We conduct phase space simulations of the positron beam using COMSOL Multiphysics® to characterize the beam properties and compare the findings with the experimentally determined energy-dependent beam diameter. To showcase the capabilities of SLOPE, we perform studies of positronium (Ps) formation on boehmite and depth-resolved coincidence Doppler-broadening spectroscopy (CDBS) of copper. In particular, the Ps formation at the hydrogen-terminated surface of boehmite is found to be maximum at a positron implantation energy of 10 eV. The range of positron energies for which we observe Ps formation agrees with the hydrogen ionization energy.

Electrochemical characterization and structural analysis of (In2O3)/(Fe2O3) nanocomposites for high-performance supercapacitors

This study presents a comprehensive investigation of the electrochemical characteristics and structural properties of novel nanocomposites with varying compositions of (In2O3)x/(Fe2O3)1-x, where x ranges from 1 to 0. These nanocomposites were synthesized using a versatile and cost-effective co-precipitation method. The crystallographic structure and morphology of the synthesized samples were thoroughly analyzed using Powder X-ray diffraction (PXRD) and Tunneling Electron Microscopy (TEM). Advanced analytical techniques were employed including Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening Spectroscopy (CDBS) to uncover critical insights into the structural and molecular properties of these nanocomposites. PALS analysis revealed valuable insights into the pore characteristics of the nanocomposites, while CDBS identified crucial molecular interactions within the materials. Electrochemical characterization of the nanocomposites is carried out using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical-impedance-spectroscopy (EIS) measurements. The (In2O3)0.3/(Fe2O3)0.7 nanocomposite exhibits a remarkable specific capacitance of 945 F g-1 at 1 A g-1 and exceptional rate performance, retaining 93.6% of its specific capacitance at a six-fold higher current density. Moreover, this nanocomposite electrode demonstrates outstanding cyclic stability, maintaining 92.1% of its specific capacitance even after 3000 GCD cycles at 8 A g-1. These findings suggest that the novel composition and integrated electrochemical properties of the (In2O3)0.3/(Fe2O3)0.7 nanocomposite hold great promise for enhancing the performance of next-generation electrochemical capacitors. This research contributes valuable insights into the design and development of advanced energy storage materials with applications in various high-performance energy storage devices.

Analysis of defects in &lt;i&gt;B&lt;/i&gt;-vacancy compensated Sm-doped PZT(54/46) ceramics and their influences on piezoelectric properties

Rare earth dopping, especially samarium (Sm) dopping is considered as an effective way to obtain high piezoelectricity by increasing local structure heterogeneity in Pb-containing &lt;i&gt;AB&lt;/i&gt;O&lt;sub&gt;3&lt;/sub&gt; perovskite ceramics. Defects play an significant role in determining piezoelectric properties in aliovalent ion doping systems. In order to obtain an insight into the effect of defects, especially &lt;i&gt;B&lt;/i&gt;-site vacancies on piezoelectricity, Sm-doped PZT(54/46) ceramics compensated by &lt;i&gt;B&lt;/i&gt;-site vacancies are fabricated by conventional solid state reaction method. The influence of defects on piezoelectric properties is studied by positron annihilation lifetime spectroscopy (PALS), coincidence Doppler broadening spectroscopy (CDBS), and conventional methods such as X-ray diffraction (XRD), scanning electron microscope (SEM), electrical performance testing on dielectricity, ferroelectricity and pizoelectricity. The XRD results show that all ceramics crystallize in a pure perovskite phase, Sm&lt;sup&gt;3+&lt;/sup&gt; doping causes a transformation from the rhombohedral to tetragonal phase and the morphotropic phase boundary (MPB) lies near Sm&lt;sup&gt;3+&lt;/sup&gt; doping content &lt;i&gt;x&lt;/i&gt; = 0.01–0.02. Electrical performance testing results indicate that with the increase of &lt;i&gt;x&lt;/i&gt;, all of the dielectricity, ferroelectricity and pizoelectricity first increase and then decrease, the sample with &lt;i&gt;x&lt;/i&gt; = 0.01 and 0.02 exhibit similar excellent dielectricity and ferroelectricity, while their pizoelectricity differs greatly, the optimal piezoelectric coefficient &lt;i&gt;d&lt;/i&gt;&lt;sub&gt;33&lt;/sub&gt; = 572 pC/N (nearly double that of undoped sample) is obtained in the sample with &lt;i&gt;x&lt;/i&gt; = 0.01. The PALS results show that Sm doping leads the defect types to change from the coexistence of &lt;i&gt;A&lt;/i&gt;-site and &lt;i&gt;B&lt;/i&gt;-site vacancies for &lt;i&gt;x&lt;/i&gt; ≤ 0.01 to mainly &lt;i&gt;A&lt;/i&gt;-site related defects for &lt;i&gt;x&lt;/i&gt; ≥ 0.02. The CDBS results further verify that the concentration of &lt;i&gt;B&lt;/i&gt;-site vacancies is highest for &lt;i&gt;x&lt;/i&gt; = 0.01 and lowest for &lt;i&gt;x&lt;/i&gt; = 0.02. It is inferred that the high pizoelectricity for &lt;i&gt;x&lt;/i&gt; = 0.01 is related to its high concentration of &lt;i&gt;B&lt;/i&gt;-site vacancies, which can dilute the number of &lt;i&gt;A&lt;/i&gt;-site vacancies and oxygen vacancies, reducing the chance of forming defect dipoles between an &lt;i&gt;A&lt;/i&gt;-site vacancy and an oxygen vacancy, facilitating domain wall motion, and enhancing piezoelectricity. This study indicates that &lt;i&gt;B&lt;/i&gt;-site vacancies can enhance piezoelectricity to some extent, which will provide some guidance for defect engineering.

Novel data analysis tool for the evaluation of Coincidence Doppler Broadening Spectra of the positron–electron annihilation line

Coincidence Doppler Broadening Spectroscopy (CDBS) of the 511keV annihilation line reveals the elemental signature at the annihilation site in matter. For this reason, CDBS enables the analysis of foreign atoms in the host matrix, vacancy-solute complexes and precipitates in solids. Due to the lack of a comprehensive program for the analysis of CDBS data we developed a modular python-based open source software solution for the reliable extraction of the annihilation line to provide so-called ratio curves highlighting the elemental signature in the spectra. In particular, the new program allows the readout and spectra averaging of multiple detector pairs. Moreover, we implemented an improved algorithm to further reduce the background by about one order of magnitude at high Doppler shifts.

Ion beam-based coincidence Doppler broadening spectroscopy of positron annihilation radiation: The experiences of Tehran Van de Graaff lab

Coincidence Doppler Broadening Spectroscopy of positron annihilation radiation is developed using high energy gamma-rays instead of standard positron source for defect assessment in thick samples. A 2.5 MeV proton beam of a Van de Graaff accelerator is used to generate high-energy gamma-ray through 19Fp,αγ16O nuclear reaction. The collimated gamma-ray is used to generate positron inside the sample. The performance of the technique to support the positron annihilation spectroscopy experiment is evaluated by investigation of the defect evolution of five low-carbon steel samples preloaded during the uniaxial tension test. The variation of the S-parameter versus strain is investigated and discussed considering the engineering stress-strain curve of the samples. Moreover, the advantages and limitations of the technique over the conventional CDBS are discussed. The developed spectrometer provides a systematic approach for the defect assessment in thick high-Z elements samples.

The influence of rhenium addition on the distribution of vacancy-type defects in tungsten

To investigate the influence of transmutation rhenium on the irradiation-induced defects in tungsten , H + and He + irradiation of 50 keV with a fixed fluence of 1 × 10 16 atoms·cm −2 were conducted on W x Re ( x =0, 3, 5 and 25 wt.%) alloys, respectively. Doppler Broadening Spectroscopy and Coincidence Doppler Broadening Spectroscopy both based on slow positron beam were employed in the current work to characterize the depth distribution and the chemical surroundings of the irradiation-induced defects. The results showed that under 723K irradiation, Re addition suppressed the accumulation of vacancy-type defects compared with pure W. This suppression was enhanced by increasing Re content in W. An obvious Re-related peak from (7-28) × 10 −3 m 0 c was observed in CDB spectra of well-annealed W-Re alloys. The irradiation effect led to the height deviation of Re-related peak. The decrease of the Re-related peak could be attributed to the formation of irradiation-induced defects in irradiated samples, and the reduction of positron annihilation fraction with core electrons of Re.

Removal of defects in CdO nanoparticle and rapid synthesis of CdO nanoflake using novel microwave technique to improve semiconductor device performance

Objective: The objective of this study is to synthesize CdO nanoparticle and nanoflakes both in a single way. Also the purpose of removing defects from semiconductor nanoparticles has been considered. Methods: Very small sized Cadmium oxide nanoparticles (11 – 29 nm) and nanoflakes of average size 70 nm were prepared by sol-gel process. Two solutions of 0.2M CdCl2 and 0.2M NaOH were used with a mixture of 1:2 ethylene glycol and methanol. Two different heating method of dried precipitate was adopted to observe the morphology and defect characteristics. Half of the precipitate was annealed in furnace and rest half in domestic microwave. Findings: The fcc (NaCl-type) structure has been found in the nanostructures with lattice constant varying with annealing temperature. We propose a linear relation for the change in lattice constant. HRTEM pictures show nanoflakes and hexagonal nanoparticles. The ratio curve of coincidence Doppler broadening spectroscopy (CDBS) found defect concentration reduction and agglomeration effect on microwave irradiation in place of furnace annealing. It indicated reduction of defects inside the nanoparticles. The S and W parameter revealed the chemical surroundings and momentum distribution of the vacancy clusters varying with crystallite size. Novelty: Furnace annealing and microwave irradiation showed different effect of surface strain on lattice. Microwave irradiation has been used to reduce defects inside CdO nanocrystals for potential application and coincidence Doppler broadening spectroscopy showed potential to be a very effective tool of defect characterization and electron momentum comparison. Also, this work presents how microwave radiation can be used to prepare nanoflakes and nanoparticles from same reactants without any substrate. Keywords: Domestic microwave; nanocrystals; positron annihilation; solgel processes

Positron Annihilation Spectroscopy of KCl (Zn) crystals

Four samples of nominally pure KCl crystals and doped with Zn2+ impurities are grown by the Czochralski method and characterized by X-ray diffraction and Proton Induced X-ray Emission techniques. Positron Annihilation Lifetime Spectroscopy (PALS) is performed to obtain information on the cation vacancy type defects and their evolution under doping. The results of the PALS experiment indicate that doping KCl by Zn2+ ions, at first increases the concentration of mono vacancies and in the second stage leads the creation of divacancy sites. Coincidence Doppler Broadening Spectroscopy (CDBS) is carried out to obtain the chemical environment of positron annihilation sites. The results of CDBS show that cation vacancies have a significant role in the annihilation process. An interesting observation is the participation of Zn2+ cations in the positron annihilation process which confirms that positrons are not completely localized on the anion sites. The internal consistency between the results of PALS and CDBS experiments is also clarified.

Porosity evaluation and positron annihilation study of mesoporous aluminum oxy-hydroxide ceramics

The pore size and type of aluminum oxy-hydroxide (boehmite) obtained using an optimized sol–gel method was investigated as a mesoporous material with the calcination temperature varied from 150 to 550 °C. Structural features were characterized by XRD, SEM, TEM, BET. Molecular interactions were identified by using PALS (positron annihilation lifetime spectroscopy) and CDBS (coincidence Doppler broadening spectroscopy). The XRD pattern showed that the sample synthesized at 250 °C has an orthorhombic phase characterizing the boehmite structure. All the samples prepared from pure aluminum metal were mesoporous according to BET results with a strong relationship between microstructure properties and temperature. The highest values of specific surface area and pore volume assigned to the sample prepared at 250 °C with 268.3 m2·g−1 and 0.313 cm3·g−1, respectively. The results of PAL-spectroscopy show that, the lifetime of positrons annihilating at pores of the boehmite at 350 °C increased as the average pore size growths because the free-volume defects in the samples increased.

Research progress of hydrogen/helium effects in metal materials by positron annihilation spectroscopy

An important feature of the irradiation process in nuclear system is the formation of large displacement cascades, in which primary knock-on atoms and secondary particles formed by nuclear reactions generate a considerable number of defects such as dislocations, vacancies and transmutation gases. Predicting and mitigating the adverse effects of damage defect and transmutation hydrogen/helium produced by high-dose neutron irradiation on the mechanical properties of structural materials is the most significant challenge facing the current development of nuclear energy. To solve this problem, understanding the interaction mechanism between hydrogen/helium atoms and micro-defects is a very important breakthrough. Precursors of helium/ hydrogen bubble, small helium/hydrogen-filled vacancy complexes, may play an important role in realizing bubble nucleation, and the formation of these complexes is affected by many factors. However, only a little information about helium/hydrogen-vacancy clusters’ behavior has been obtained in metal/alloy materials. This is mainly limited by the characterization methods, such as the limited resolution of transmission electron microscope (TEM). Helium/hydrogen-vacancy clusters cannot be observed by TEM before the formation of helium bubbles. Applications of positron annihilation to the study of crystal lattice defects started around 1970s, when it was realized that positron annihilation is particularly sensitive to vacancy-type defects and that annihilation properties manifest the nature of each specific type of defect. In recent years, with the continuous development of slow positron beam and the improvement of various experimental testing methods based on slow positron beam, the application of positron annihilation technology has been extended to the research field of hydrogen/helium behavior in metal materials, which plays an important role in studying the hydrogen/helium radiation damage to metal materials. In this review, the basic principles of positron annihilation spectroscopy are briefly discussed and the three most important measurement methods used for hydrogen/helium effect studies are described (i.e. positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS), coincidence Doppler broadening spectroscopy (CDBS)). In this paper, the application of positron annihilation spectroscopy to the study of hydrogen/helium behavior in metal materials is reviewed in combination with the reported relevant developments (including our research group’s achieve-ments). The advantages of three commonly used measurement methods in the following specific studies are highlighted: 1) The estimation of bubble size and concentration; 2) irradiation damage induced by hydrogen/helium; 3) the evolution behavior of irradiation-induced defects in the heat treatment process; 4) sy-nergistic effect of hydrogen and helium.

Structural defects characterization of silver-phosphate glass nanocomposites by positron annihilation and related experimental studies

The structural defects and their role in the formation and properties of the metal oxide glass nanocomposite xAgI–(1-x) (0.65Ag2O–0.35P2O5) have been studied using several sensitive spectroscopic techniques. The nanocomposite samples have been prepared by conventional melt-quenching method and the formation of nanocrystallites over the amorphous glassy matrices has been verified from X-ray diffraction and transmission electron microscopy studies. The optical band gap energies were determined from Tauc's plots of UV–Vis absorption spectra taken from a spectrophotometer. Special emphasis on the defect characterization and defects-mediated stages of evolution is made possible through positron annihilation lifetime (PAL) and coincidence Doppler broadening spectroscopic (CDBS) measurements, which further helped in the investigation of the growth of defects, especially the Ag+ ion vacancies, within the nanocomposites. A defect-specific positron lifetime is identified to represent positron trapping in the interfacial gaps between nanocrystallites and the amorphous glass matrices besides the vacancies of Ag+ ions. The longest positron lifetime component occurs due to the formation of orthopositronium in free volume holes of small sizes and it increased as an effect of agglomeration of the free volume holes within the amorphous glassy matrices at increased incorporation of AgI. The CDB spectra revealed the changing proximities of the Ag+ ion vacancies to the surrounding oxygen ions. The positron annihilation characteristics were especially indicative of the importance of the choice of stoichiometry in the formation of the nanocomposite and its exhilarating properties.

Momentum distribution of core and 3d electrons in mechanically strained NiO nanoparticles

Nickel oxide (NiO) nanoparticles was prepared using top-down method in the size range 12–70 nm via ball milling of their bulk. Bulk size NiO was prepared by heating of Ni(NO3)2.6H2O in air. This NiO powder was then milled in planetary ball mill keeping a fixed powder to ball ratio. Strain was introduced in the powder during milling process. The structure of milled samples was studied using x-ray diffraction (XRD) while Vibrating Sample Magnetometer (VSM) measurements were carried out to study magnetization in samples. The peaks in the x-ray diffraction (XRD) pattern correspond to pure NiO phase showing no trace of any other phase. Strained particles showed higher magnetization as compared to relaxed NiO nanoparticles. Positron Annihilation Coincidence Doppler Broadening (PACDB) spectroscopy was used to study the 3d and core electron momentum distribution in the vicinity of Ni atom. It is observed that as milling proceeds and the particles become more and more laminar the positrons get annihilated preferentially at the oxygen core. A relation between PACDB quotient ratio and mechanical strain is established in this study.

Investigation of cation vacancies in Zinc substituted maghemite by positron annihilation lifetime and Doppler broadening spectroscopy

In this work we investigated vacancies in maghemite and Zn substituted maghemite (γ-Fe2−yZn3y/2O3, y=0.0, 0.11, 0.24, 0.36, 0.50 and 0.66) nanoparticles using coincidence Doppler broadening and positron annihilation lifetime spectroscopy. Coincidence Doppler broadening spectroscopy (CDBS) measurements showed that the positrons annihilated in cation vacancies that surrounded by oxygen anions. Also, the CDBS showed that in pure maghemite and in the y=0.11 samples the vacancies are in octahedral and tetrahedral sites, respectively. For other samples they are distributed in both octahedral and tetrahedral sites. The positron annihilation lifetime spectroscopy (PALS) measurements confirmed the results of the CDBS measurements and also exhibited that the number of vacancies in y=0.36 sample is less than the other samples. This is attributed to Zn substituted magnetite phase in this sample as well as Zn substituted maghemite phase.

Investigation of Ion-Mediated Charge Transport in Methylammonium Lead Iodide Perovskite

We have investigated the origin of ionic conductivity in methylammonium lead iodide (MAPbI3) by positron annihilation lifetime spectroscopy (PALS), supplemented by coincidence Doppler broadening spectroscopic (CDBS) techniques which reveal the presence of methylammonium (MA+) defects in the perovskite crystal lattice. Crystallinity and the defect concentration vary with the perovskite synthesis process, which in turn governs the magnitude of ionic conductivity. Single-crystalline perovskite contains lesser defects with equal probability of developing both cationic and anionic (halide) vacancies, whereas the polycrystalline perovskite sample developed through mechanical process carries mainly cationic, i.e., MA+ vacancy (V′MA) in its crystal lattice as indicated by direct current (dc) polarization experiment.

The ability of the Coincidence Doppler Broadening Spectroscopy to characterize polymers containing different chemical elements

Hydrocarbon polymers, O-containing, F-containing and Cl-containing polymers are comprehensively studied by Coincidence Doppler Broadening Spectroscopy (CDBS). It is shown that for polymers with different chemical structure, CDBS results can effectively distinguish polar groups CO, CCl, and CF. For polymers with similar chemical structure, the intensity of the element-specific peak in the CDBS ratio curve is dependent not only on the fraction of free positrons, but also on the content of characteristic atom in polymer repeated unit, and the polarity of the polymer molecule. For polymers containing several different polar groups, such as PCTFE (CF & CCl) and PFA (CF & CO), whether the element-specific peak appears or not depends on the amount of the polar groups and its positron capture ability. This work may provide insights into potential applications of CDBS for studying complex polymer systems.

First-cycle defect evolution of Li1−xNi1/3Mn1/3Co1/3O2 lithium ion battery electrodes investigated by positron annihilation spectroscopy

Abstract In this study the structure and evolution of vacancy type defects in lithium ion batteries are investigated in respect of crystallographic properties. The relation between positron annihilation and electronic structure is discussed in terms of structural dynamics during the lithiation process. Samples of Li1−xNi1/3Mn1/3Co1/3O2 (NMC-111) electrodes with decreasing lithium content (x = 0–0.7) covering the whole range of state of charge were electrochemically prepared for the non-destructive analysis using positron coincidence Doppler broadening spectroscopy (CDBS). The positron measurements allowed us to observe the evolution of the defect structure caused by the delithiation process in the NMC-111 electrodes. The combination of CDBS with X-ray diffraction for the characterization of the lattice structures enabled the analysis of the well-known kinetic-hindrance-effect in the first charge-discharge cycle and possible implications of vacancy ordering. In particular, CDBS revealed the highest degree of relithiation after discharge to 3.0 V at 55 °C. For the first time, we report on the successful application of CDBS on NMC-111 electrodes yielding new insights in the important role of defects caused by the delithiation process and the kinetic hindrance effect.

Positron annihilation and magnetic properties studies of copper substituted nickel ferrite nanoparticles

Single phase copper substituted nickel ferrite Ni1-xCuxFe2O4 (x=0.0, 0.1, 0.3 and 0.5) nanoparticles were synthesized by the sol–gel method. TEM images of the samples confirm formation of nano-sized particles. The Rietveld refinement of the X-ray diffraction patterns showed that lattice constant increase with increase in copper content from 8.331 for x=0.0 to 8.355Å in x=0.5. Cation distribution of samples has been determined by the occupancy factor, using Rietveld refinement. The positron lifetime spectra of the samples were convoluted into three lifetime components. The shortest lifetime is due to the positrons that do not get trapped by the vacancy defects. The second lifetime is ascribed to annihilation of positrons in tetrahedral (A) and octahedral (B) sites in spinel structure. It is seen that for x=0.1 and 0.3 samples, positron trapped within vacancies in A sites, but for x=0.0 and 0.5, the positrons trapped and annihilated within occupied B sites. The longest lifetime component attributed to annihilation of positrons in the free volume between nanoparticles. The obtained results from coincidence Doppler broadening spectroscopy (CDBS) confirmed the results of positron annihilation lifetime spectroscopy (PALS) and also showed that the vacancy clusters concentration for x=0.3 is more than those in other samples. Average defect density in the samples, determined from mean lifetime of annihilated positrons reflects that the vacancy concentration for x=0.3 is maximum. The magnetic measurements showed that the saturation magnetization for x=0.3 is maximum that can be explained by Néel’s theory. The coercivity in nanoparticles increased with increase in copper content. This increase is ascribed to the change in anisotropy constant because of increase of the average defect density due to the substitution of Cu2+ cations and magnetocrystalline anisotropy of Cu2+ cations. Curie temperature of the samples reduces with increase in copper content which can be explained based on Néel’s theory.

Formation and annihilation of positronium in silica aerogels under atmosphere of oxygen and nitrogen mixture

Formation and annihilation of positronium in silica aerogels under flowing nitrogen and oxygen mixtures with various ratios were studied by positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The annihilation rates, effective cross section for o-Ps quenching on oxygen molecules and effective number of electrons per oxygen molecule available for 2γ-annihilation of positron in o-Ps have been measured by PALS. It is found that the longest o-Ps lifetime decreases remarkably and the center of positron 2γ-annihilation peak becomes narrower and narrower as the oxygen partial pressure increases. The results show o-Ps quenching on oxygen molecules occurs through spin conversion, which simultaneously results in a remarkable decrement in its intensity. However, the total Ps formation probability is enhanced with increasing oxygen partial pressure, which is interpreted for higher probability of Ps formation on an oxygen molecule.

Positron annihilation spectroscopic studies of multiferroic Bi1-xPrxFeO3 nanocrystalline compounds

Positron lifetime (PL) and coincidence Doppler broadening spectroscopic (CDBS) experiments were carried out on BiFeO3 samples doped with Pr(in place of Bi) in concentrations (x) = 0, 0.05, ..., 0.35. In the initial stages of doping, the existing Bi3+ vacancies are occupied by Pr3+ ions and positron trapping is reduced. From x = 0.15 to 0.35, more number of positrons diffused to the surfaces of the nanocrystallites. The CDBS ratio curves showed enhancement of the peak at pL = 10.2×10-3 m0c due to positron annihilation with the 2p electrons of oxygen. It suggested increased trapping of positrons in newly created cationic vacancy-type defects due to the mismatch of ionic radii of Pr3+ substituting Bi3+ ions. The lattice parameters decreased (3.9661-3.9248Å) while the nanocrystallitesizes reduced and increased (28.6-34.2nm) during the doping.